Having tried to observe this over a long period, I'm gonna generalize: Usually to my eyes, there are about three or four top tier FIs (85+) who are open for business every season. You miss out on them, you have to settle for someone in the mid 70s which - shouldn't? - hurt your prospects but maybe won't help them either. And if you're using an FI lower than that hopefully you're trying to develop your own. (I did that once, found a minor league coach with Fielding in the upper 50s and stashed him at 1B Coach until his rating got into the 70s.)
Point being that if I want to make sure I get one of those top three available, sure I'll go to 5 mil.
This all depends on your world and having an idea what the other owners are likely to pay.